Analog to digital converters usually require precision resistors or capacitors and switches. Such elements tend to unduly increase the cost of those converters as does the need for large power supplies. In applications using microprocessors the possibility of using the microprocessor for counting time presents itself. It is therefore an object of this invention to provide apparatus for producing in a simple, accurate and economical way a digital representation of an analog voltage when a microprocessor is available to provide the counting function.